[LEAPSECS] LEAPSECS Digest, Vol 45, Issue 1

[Tony Finch]

On Fri, 3 Sep 2010, Zefram wrote:

> Tony Finch wrote:

> >Thanks for the informative explanation, but GMT is not and was not UT1.

>

> Picky, picky.  OK, let's look at the strictest sense of "GMT", taking the

> Greenwich meridian to be defined by the Royal Observatory, Greenwich,

> rather than by the ITRF.  Specifically, the 1851 meridian defined by

> Airy's transit circle, adopted as the prime meridian by the International

> Meridian Conference in 1884.


Yet more interesting stuff, thanks again!

Another thing to consider is that the last practical astronomical
realisation of GMT was carried out at Herstmonceaux not Greenwich, and
some errors were introduced when time was transferred from one to the
other which is part of the reason for the discrepancy between WGS84 and
the Airy circle. Also I understand that GMT included some corrections
similar though not identical to UT2 - see also Steve's little history he
sent yesterday which said CCIR rec. 374 required time signals to be within
100ms of UT2 between 1963 and 1970.

But this is mostly beside the point, which is that there is no
organization maintaining and promulgating an official astronomical GMT.
You could create a close approximation to an astronomical GMT but it would
have no official force. As we have seen there are a lot of intricate
details whose necessity people can legitimately disagree about and no way
to determine an official consensus. Which is why I say that astronomical
GMT doesn't exist.

Tony.
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